Re: Valid Attribute and Unchecked Conversion

[ka@socrates.hr.att.com (Kenneth Almquist)]

kst@thomsoft.com (Keith Thompson) writes:
> RM95-13.9.1(12) says:
>
>	 A call to an imported function or an instance of
>	 Unchecked_Conversion is erroneous if the result
>	 is scalar, and the result object has an invalid
>	 representation.
[snip]
> So, suppose I have a sparse enumeration type:
>
>     type Enum is (Ten, Twenty, Thirty);
>     for Enum use (10, 20, 30);
>     for Enum'Size use 8;
>
> and a corresponding integer type:
>
>     type Int_Type is range 0 .. 31;
>     for Int_Type'Size use 8;
>
> Given an arbitrary value of type Int_Type, is there any non-erroneous
> way to get the corresponding Enum value if there is one, or raise
> an exception if there isn't?  By "corresponding", I mean 10 => Ten,
> 20 => Twenty, 30 => Thirty.

Do an unchecked conversion to a nonscalar type.

-- Convert Int_Type to Enum, raising constraint error if the result is
-- invalid.
function To_Enum(I : Int_Type) return Enum is
    type Wrapped_Enum is record
        Value : Enum;
    end record;
    for Wrapped_Enum'size use Enum'size;

    function To_Wrapped_Enum is new
        Ada.Unchecked_Conversion(Int_Type, Wrapped_Enum);
 
    W : Wrapped_Enum;
begin
    W := To_Wrapped_Enum(I);
    if W.Value'valid then
        return W.Value;
    else
        raise Constraint_Error;
    end if;
end To_Enum;